1. Field of the Invention
The present invention relates to a data signal timing correction device, a filter device, and a wireless portable communication terminal device, and more particularly, is suitably applied to a data signal timing correction device, a filter device, and a wireless portable communication terminal device which are used in a wireless communication system to communicate through a transmission line of which a usable frequency band is limited.
2. Description of the Related Art
In this type of wireless communication terminal device, a data signal to be transmitted (hereinafter, referred to as a baseband signal) is generated as a digital signal. The baseband signal is limited within a prescribed frequency band by a filtering processing, and then is converted into an analog signal. A carrier wave is modulated with the baseband signal converted into the analog signal, and then the modulation signal is transmitted through a prescribed transmission line of which a usable frequency band is limited.
FIG. 1 generally shows a circuit configuration of a transmission filter device and its vicinity provided in a wireless communication terminal device such as a portable telephone. A clock signal S1 generated at an oscillator 1 is supplied to a transmission filter 2 as a reference clock. Further, a transmission symbol generating unit 3 generates a baseband signal S2 to be transmitted and sends it to the transmission filter 2. Note that, in a CDMA-PCS which is an information communication system using portable telephones and which has been proposed in the United States, frequency of the baseband signal S2 is regulated as 1.2288 [MHZ] by J-STD-008 of the ANSI standard, and the wireless communication terminal device based on the standard generates the baseband signal S2 with the frequency. Also, frequency of the clock signal S1 is generated with 4.9152 [MHZ] which is four times as much as the baseband signal S2 because the transmission filter 2 performs a quadruple-oversampling on the baseband signal S2. In this connection, the frequency of the clock signal S1 is not only limited to 4.9152 [MHZ] but also it can be generated in accordance with a magnification of oversampling the baseband signal S2.
The transmission filter 2 is comprised of a resampler 4, a digital filter 5, and a digital-to-analog converter (hereinafter, referred to as D/A converter) 6, and drives based on the clock signal S1 which is supplied from the oscillator 1. The baseband signal S2 which is supplied from the transmission symbol generating unit 3 is inputted to the resampler 4. The resampler 4 performs the oversampling processing on the baseband signal S2 with the clock signal S1 having an integral multiple frequency of the baseband signal S2. The resampler 4 sends out a pulse signal S3 obtained by the oversampling processing, to the digital filter 5. The digital filter 5 quantizes the pulse signal S3 in accordance with a prescribed frequency characteristic. The digital filter 5 sends out a quantization signal S4 obtained by the quantization, to the D/A converter 6. The D/A converter 6 converts the quantization signal S4 into a transmission signal S5 being an analog signal. Then, the wireless communication terminal device extracts only a prescribed frequency band from the transmission signal S5 by an analog low-pass filter (not shown) to transmit it.
More specifically, as shown in FIG. 2A, the baseband signal S2 generated at the transmission symbol generating unit 3 is a train of impulses of 1.2288 [MHZ], and when observing the baseband signal S2 on the frequency axis, one band having 1.2288 [MHZ] width appears every 1.2288 [MHZ] repeatedly.
As shown in FIG. 2B, the resampler 4 widens one band of the baseband signal S2 to 4.9152 [MHZ] width by oversampling the baseband signal S2 with 4.9152 [MHZ] frequency which is a quadruple frequency of 1.2288 [MHZ]. The one band having 4.9152 (MHZ) width contains four pieces of information being the original 1.2288 [MHZ]. More precisely, in the oversampling processing, three pieces of information which is "0" are filled between trains of impulses of the baseband signal S2 (points between impulses in FIG. 2B).
As shown in FIG. 2C, a band of the pulse signal S3 obtained by oversampling is narrowed by the digital filter 5. The quantization signal S4 obtained by the band control is sent out via the D/A converter 6, so that the transmission filter 2 sends out a signal for forming an envelope curve shown in FIG. 2C. The signal sent from the transmission filter 2 is outputted through the analog low-pass filter, and thereby the wireless communication terminal device extracts and outputs only a prescribed frequency band.
The wireless communication terminal device can widen the interval between frequency bands of the baseband signal S2 by performing the oversampling processing with a prescribed magnification at the transmission filter 2, and can easily extract the prescribed frequency band by the analog low-pass filter without utilizing highly efficient frequency characteristic. Thus, in the wireless communication terminal device, a load can be reduced by decreasing the characteristic required for the analog low-pass filter.
Further, in the wireless communication system to communicate between the wireless communication terminal device and the base station by radio as represented by a cellular system, the wireless communication terminal device synchronizes transmission timing of a transmission signal with the other party of communication. Especially, in a system using a code division multiple access (CDMA) scheme as a multiplexing system of a transmission signal, highly efficient timing synchronization becomes prerequisite.
In FIG. 3, the same reference numerals are applied to correspond to FIG. 1, numeral 10 generally shows a wireless communication terminal device according to the CDMA scheme. The wireless communication terminal device receives a signal transmitted from the other party via the base station and moreover, transmits the transmission signal S5 to the other party at transmission timing determined according to timing of a reception signal.
In the wireless communication terminal device 10, a reception signal S6 being an analog signal is inputted to an analog-to-digital converter 11 (hereinafter, referred to as A/D converter 11) to be converted into a digital signal S7. The A/D converter 11 outputs the obtained digital signal S7 to a demodulator 12. The demodulator 12 demodulates the digital signal S7 modulated by a prescribed modulation form at the transmission side, and outputs it to a reception symbol processing unit 13 as a reception symbol S8 which is packet data in which information is stored. In addition, in this time, the demodulator 12 detects the timing of the digital signal S7 and outputs a control signal S9 having a voltage level corresponding to the detection result. The reception symbol processing unit 13 converts the reception symbol S8 into an output signal such as an audio signal to output it. Note that, the A/D converter 11, the demodulator 12, and the reception symbol processing unit 13 are supplied with the clock signal S1 from the oscillator 1, and drive at the timing the clock signal S1 as a reference clock.
On the other hand, a transmission clock generating unit 14 generates a reference clock signal S10 with a frequency corresponding to the voltage level of the control signal S9 to supply it to the transmission symbol generating unit 3 and moreover, generates a reference clock signal S11 having an integral multiple frequency of the reference clock signal S10 to supply it to the transmission filter 2. More precisely, the transmission clock generating unit 14 is comprised of a voltage controlled oscillator (VCO) and changes the frequency of the reference clock signal S10 to be generated in accordance with the voltage level of the control signal S9. Note that, for the frequency of the reference clock signal S11, an integral multiple frequency of the reference clock signal S10 is selected because of oversampling the baseband signal S2.
The transmission symbol generating unit 3 generates the baseband signal S2 which is a transmission symbol, on the basis of the reference clock signal S10, and sends it to the transmission filter 2. The baseband signal S2 is inputted to the resampler 4 to be oversampled with a magnification based on the frequency of the reference clock signal S11. The pulse signal S3 which is output from the resampler 4 is supplied to the digital filter 5, and the band of the pulse signal S3 is narrowed. The quantization signal S4 obtained by narrowing the band is converted into the transmission signal S5 being an analog signal via the D/A converter 6. The wireless communication terminal device 10, after sending out the transmission signal S5 from the transmission filter 2, extracts and outputs only the prescribed frequency band by the analog low-pass filter (not shown).
As described above, the wireless communication terminal device 10 synchronizes the timing to transmit the transmission signal S5 with the timing which is obtained from the reception signal S6. Accordingly, the wireless communication terminal device 10 extracts timing information from the digital signal S7 which is obtained by A/D converting the reception signal S6, by the demodulator 12 to detect whether or not the difference between the timing information and the timing information in the wireless communication terminal device 10 exists. As a result of the detection, when it is determined that the difference between the timing information which is obtained from the reception signal S6 and the internal timing information exists, the wireless communication terminal device 10 adjusts the internal timing to correct the difference. More precisely, the wireless communication terminal device 10 temporarily changes frequencies of the reference clock signals S10 and S11 which are generated by the transmission clock generating unit 14 from the control signal S9 which is outputted from the demodulator 12, in order to synchronize the internal timing with the timing information which is obtained from the reception signal S6.
FIG. 4, in which the corresponding parts of FIG. 3 are designated with the same reference numerals, generally shows the internal configuration of the demodulator 12, and that the input digital signal S7 is supplied to demodulation units 15 to 17 and a synchronous trapping/tracking circuit 18. The synchronous trapping/tracking circuit 18 detects an optimal phase of prescribed diffusion lines to be combined with the digital signal S7, and informs each of the demodulation units 15 to 17 of the phase. Each of the demodulation units 15 to 17 combines the digital signal S7 with the prescribed diffusion lines by using the phase detected at the synchronous trapping/tracking circuit 18.
More specifically, the reception signal S6 which is received at the wireless communication terminal device 10 is a signal generated by combining an information signal with the diffusion lines at the transmission side, and its own signal of which the amplitude is decreased and the phase is shifted is added to the original transmission signal by the action of multi-pass on the transmission line. Therefore, the demodulator 12 combines the same diffusion lines as those combined at the transmission side with the digital signal S7 while shifting the phase in the synchronous trapping/tracking circuit 18. When the phase of diffusion lines to be combined while shifting is equal to the phase of diffusion lines combined at the transmission side, the value of correlation appears as a peak point. Thus, by combining the phase of diffusion lines while shifting, the peak point having the highest correlation value can be easily detected and the optimal phase of the diffusion lines to be combined with the digital signal S7 can be obtained.
Further, since the reception signal S6 which is received by the wireless communication terminal device 10 includes a plurality of signals of which the phases are shifted from each other by the action of the multipath as described above, a plurality of peak points of the correlation value will be obtained in the case where the diffusion lines are combined while shifting the phases.
The synchronous trapping/tracking circuit 18 selects three phases capable of obtaining especially high peak values from a plurality of thus obtained peak points, and respectively distributes and informs the phases to the demodulation units 15 to 17 as phase information signals S12. Each of demodulation units 15 to 17 demodulates the digital signal S7 by combining the diffusion lines with the informed phase to output the obtained information signal to a signal combining unit 19. The signal combining unit 19 sets the phase of the information signals which are supplied, combines the information signals by weighting according to the strength of each signal, and outputs it as the reception symbol S8.
Further, there are cases where a part of a transmission path is cut off due to the transfer of the wireless communication terminal device, and thereby out of the detected three phases capable of obtaining especially high peak values, some phases drop to the undetectable level. The synchronous trapping/tracking circuit 18 detects the phase capable of constantly obtaining the high peak value, and if the phase which is used for demodulating the digital signal S7 at the demodulation unit 15, 16 or 17 drops to the undetectable level, the synchronous trapping/tracking circuit 18 newly selects the phases capable of obtaining the high peak value and respectively informs the demodulation units 15 to 17 of the phases again.
Further, in the wireless communication terminal device according to the CDMA scheme, the timing of transmitting the transmission signal S5 is determined based on the timing of the first arrived phase out of the three phases capable of obtaining the high peak value. Here, there is a case where the timing of the first arrived phase capable of obtaining an especially high peak value varies momentarily due to the cutoff of a transmission path or soft hand off. In this case, the wireless communication terminal device 10 (FIG. 3) newly detects the timing of the first arrived phase capable of obtaining the high peak value at the synchronous trapping/tracking circuit 18, and outputs the control signal S9 to the transmission clock generating unit 14 in accordance with the detected timing. The transmission clock generating unit 14 controls the timing of transmitting the transmission signal S5 by speeding up or delaying the frequencies of the reference clock signals S10 and S11 on the basis of the control signal S9.
The wireless communication terminal device 10 synchronizes with the timing at which the other party transmits the transmission signal S5 by determining the transmission timing of the transmission signal S5 based on the timing of the reception signal S6. In the case where the transmission timing of the transmission signal S5 varies due to the cutoff of the transmission path, the wireless communication terminal device 10 can continuously secure the synchronization of transmission timing by adjusting the frequencies of the reference clock signals S10 and S11 in accordance with the timing which is newly obtained based on the reception signal S6.
According to the foregoing construction, the wireless communication terminal device 10 determines the transmission timing of the transmission signal S5 at the timing which is obtained from the reception signal S6 as described above, and controls the frequency of the reference clock signal S10 generated by the transmission clock generating unit 14 accordingly in order to generate the baseband signal S2. In the case of changing the transmission timing according to the control, the demodulator 12 changes the voltage level of the control signal S9 to be output in order to supply the control signal S9 to the transmission clock generating unit 14 which is the VCO. Thus, the frequencies of the reference clock signals S10 and S11 which are generated by the transmission clock generating unit 14 can be controlled and the transmission timing can be synchronized with the timing of reception signal S6.
However, since output of the VCO generally varies greatly because of the small change of voltage level, there is a problem that the extremely high accuracy is required for controlling the voltage level.
Further, in the case where the frequency of the reference clock signal S10 is controlled and changed in order to synchronize with the timing of the reception signal S6, the transmission symbol generating unit 3 changes the time range per one symbol unit of the baseband signal S2. For example, in case of speeding up the transmission timing, the transmission symbol generating unit 3 temporarily shortens the time range per one symbol of the baseband signal S2 with respect to the timing of clock signal S1. On the other hand, in the case of delaying the transmission timing, the transmission symbol generating unit 3 temporarily widens the time range per one symbol. The wireless communication terminal device 10 changes the transmission timing of the transmission signal S5 in this way.
However, thus generated baseband signal S2 includes jitter elements, so that there is a problem that spectrum distortion occurs in the transmission signal S5.
Furthermore, in the case of changing and synchronizing the transmission timing of the transmission signal S5 with the timing which is newly obtained based on the reception signal S6 due to the cut off of the transmission path, it is considered that the new transmission timing varies rapidly and greatly as compared with the former timing. In addition, in the case of transmitting a carrier wave which is modulated with the transmission signal S5 generated based on greatly changed timing, there is a problem that the base station through which the carrier wave is transmitted from the transmission side to the reception side cannot follow the greatly changing transmission timing.